Electrical servo systems are widely utilized for many different applications in which position control is necessary; typically, they are used to drive robots and X-Y tables, to position mirrors for laser applications and gun turrets, in integrated circuit production equipment, and the like. Primary criteria for such systems are of course high precision and responsiveness to input commands, as well as economy and facility of manufacture, reliability and durability.
Stepper motors have been used for such motion control applications, and are advantageous from the standpoint of providing high stiffness and positioning capability, coupled with comparatively low cost and relative simplicity. However, they do not generally offer optimal dynamic characteristics and, because of the absence of feedback capability, elements of uncertainty are inherent.
Closed loop DC servo motor systems provide considerable dynamic performance benefits, including speed range, acceleration, torque-to-inertia ratios, and frequency response; however, they tend to be deficient in static performance characteristics. Moreover, the feedback devices employed in such servo motor systems for the extraction and utilization of information for commutation, and for speed and position control (e.g., shaft angle position detectors, tachometers, encoders and resolvers), are expensive, tend to be fragile in some instances, and may give rise to unreliability. In general, systems of this kind are relatively complex and troublesome to install and maintain.
Accordingly, it is a broad object of the present invention to provide a novel motion control system, and a novel brushless DC motor for use therein, which are relatively simple and inexpensive to manufacture, which afford a highly desirable balance of accuracy, speed and torque characteristics, and which are highly effective, efficient and reliable for their intended purposes.
A more specific object is to provide such a brushless DC motor which operates in a closed loop mode, without need for any added feedback device, and in which the magnitude and accuracy of the feedback signal is maximized.
Additional objects of the invention are to provide a motor having the foregoing features and advantages, which is highly responsive to commands, which enables extraction of exact rotor position information at standstill and has a large number of angular resolution points to provide very precise position information, and which has very smooth running, and full power starting, torque characteristics.
The prior art shows a wide variety of systems and motors, some of which may be employed for motion control applications and may have certain of the features hereinabove discussed, as indicated by the following U.S. patents:
Polakowski, No. 3,453,512 provides a brushless DC motor which employs silicon controlled rectifiers in the armature switching circuits, the turn-on signal being generated by an angular position detector and the turn-off signal being generated by a capacitor. The position detector may consist of a series of stationary coils which are sequentially inductively coupled, by a member mounted to rotate with the field structure, with a common coil.
In Veillette, No. 3,501,664, a system is disclosed for regulating a DC motor having an internal stator field that is rotated in space 90.degree. ahead of the rotor field. The stator has teeth on its inner periphery arranged in non-diametrically oriented pairs, which carry secondary windings through which current, applied through a primary winding on the main body of the stator, is transferred sequentially as the rotor poles align with the teeth during rotation. As illustrated, the teeth of the stator are identical, and taper in the radially inward direction.
The DC motor taught by Lahde, No. 3,541,407 utilizes two-terminal field coils as both a pickup, to sense the position of the rotor, and also as a power coil to provide driving torque.
Kobayashi et al, No. 3,590,353 shows an electronically commutated motor having an outside and an inside rotor positioned on a common shaft, the inside rotor serving a position detecting function. Primary and secondary windings on an internal detecting stator are variably electromagnetically coupled, depending upon the position of the rotor.
A phonograph turntable, driven directly by an electrically controlled, variable speed brushless DC motor, is shown in Kobayashi et al, No. 3,683,248. Winding pairs within the stator are selectively coupled, depending upon the position of an internal position detector rotor, to control current flow through particular outer stator windings to drive an external rotor.
In Coupin et al, No. 3,794,895, an electronically commutated DC motor is described in which the stator is wound with pairs of power and detector coils, the latter providing a speed-dependent signal which is dephased by 90.degree. for control of the power amplifier.
A self-exciting DC motor, having means for preventing rotation in one direction, is disclosed in Kanamori No. 3,903,463. The stator poles are wound with both field and armature coils, and the position detecting elements include cores that are magnetically saturated by a permanent magnet and high frequency coils.
Machida No. 3,997,823 teaches a circuit for a brushless DC motor, in which the stator employs star-connected fixed windings. Means is provided for detecting position signals induced in the fixed windings by rotation of the rotor, which signals are employed to control switching means for supplying driving current to at least one of the windings.
In Gosling et al, No. 4,096,420, an oscillator with an LC resonance circuit is employed in the control circuit for a brushless DC motor, oscillation of the oscillator being modulated in response to induction caused in a sensing coil by the rotor field.
Wright, No. 4,162,435, discloses a circuit for a brushless DC motor, wherein the voltage induced across one unenergized winding is sampled, integrated and compared to a predetermined, position-indicating voltage to derive a control signal while at least one other winding is energized, for selective commutation.
A commutatorless DC motor drive system is provided by Gelenius, No. 4,262,237, in which a permanent magnet rotor induces AC potential waveforms in phase displaced stator phase windings. Means is provided for initiating rotor rotation from standstill, to initially induce the potential waveforms in the stator phase windings, means is provided for producing a switch point reference signal, and means responsive to the induced waveforms is provided for sustaining rotor rotation by sequentially completing and interrupting individual stator phase winding energizing circuits, in controlled relation to the reference signal.
Dittman et al, No. 4,297,622, discloses a two phase gyrosystem which employs two series-connected, motion-sensing reference coils, located 180.degree. apart, for motor drive and control.
A brushless DC motor is disclosed in Muller, No. 4,481,440 which utilizes a permanent magnet rotor in which the poles, viewed in the direction of rotation, have approximately rectangular or trapezoidal magnetization curves. The harmonic fields included in such poles induce voltages in a sensor winding of the stator which corresponds to the harmonic wave for which the winding is dimensioned.
A control device for a brushless DC motor is taught in Tokizaki et al, No. 4,495,450. It has a rotor position detecting circuit in which voltages induced in stator coils by rotation of the rotor are compared to neutral voltage at a virtual neutral point to detect polarity changing points. Based thereupon, an inverter controls the conducting modes of the stator coils to control rotation of the motor.
The DC motor of Rhee, No. 4,551,658 includes brushes and a commutator. After starting, the brushes are centrifugally displaced from the commutator to break the starting circuit.
In June of 1985, an article entitled "Multiple-Pole Stepping Motor" was published. It describes a hybrid stepping motor wound for two-phase operation, wherein every two adjacent poles of each group are connected in series, but for opposite polarity.